1. Field of the Invention
The present invention relates to a phase-change optical recording medium for recording information by reversibly changing the state of a recording film by irradiation with a light beam, more particularly, to a phase-change optical recording medium in which atomic arrangement of the recording film is changed between an amorphous state and a crystalline state.
2. Description of the Related Art
The principle of recording, erasure and reproduction in a phase-change optical recording medium is as follows. When a portion of a phase-change recording film is heated by irradiation with light to a temperature higher than the melting point thereof, the heated portion is melted. Then, the melted portion is rapidly cooled, the portion becomes to have amorphous atomic arrangement, with the result that information is recorded in that portion. Now, if the phase-change recording film is retained for a prescribed time or more within a temperature range lower than the melting point and higher than the crystallizing temperature, the phase-change recording film is crystallized in the case where the film assumes an amorphous state in the initial stage, though the crystalline state is maintained in the case where the film assumes a crystalline state in the initial stage. Thus, erasure is performed under the particular conditions. In such a phase-change recording film, the reflected light intensity from the amorphous portion differs from that from the crystalline portion. It follows that it is possible to read the recorded information by converting the reflected light intensity into electric signals, followed by analog to digital conversion.
In order to increase the recording density of the phase-change optical recording medium, it is conceivable to employ two approaches given below. One approach is to reduce a recording mark pitch in the track direction. In this case, however, since the recording mark is rendered smaller than the reproduction beam size when the pitch reduction is advanced, two recording marks may be temporarily included in the reproducing beam spot. Therefore, signal amplitude is lowered in the case where the recording marks are positioned very close to each other, though significantly high signal amplitude can be obtained in the case where the recording marks are positioned sufficiently away from each other. When signals with low amplitude are converted into digital data, errors are likely to be generated.
Another approach for increasing the recording density is to reduce a track pitch. This approach makes it possible to increase the recording density without significantly lowering the signal intensity, unlike the case of reducing the recording mark pitch noted above. Where the track pitch is reduced to a level substantially equal to or smaller than the beam size, however, this approach would bring about a problem of the so-called “cross-erase”, that is, the information recorded in a certain track would be deteriorated in a process of writing or erasure in the adjacent tracks.
Therefore, it is necessary to overcome the above-noted problems in order to increase the recording density of the phase-change optical recording medium.
In the optical disk drive system for the next generation DVD, it is a target to overcome the problems noted above by irradiating the phase-change optical recording medium with a laser beam having a beam size condensed to a level close to the diffraction limit. Various proposals have been made along this line. However, it cannot be expected to optically reduce the beam size further, as far as visible light is used. It has also been studied to use near-field light in an attempt to reduce the beam size further. However, various problems are left unsolved in the optical recording medium using the near-field light and, thus, such a medium has not yet entered the stage of the practical application. Such being the situation, in order to improve the recording density, it is considered as the best approach to decrease the cross-erase in place of reducing the beam size.
The cross-erase noted above would be brought about by two causes. One of the causes is that, when an adjacent track to a track in question is irradiated with a laser beam, the foot portion of the laser beam overlapping the track in question has a considerably high intensity, with the result that the recording mark in that track is deteriorated by only the effect of the light application. Another cause is that, when an adjacent track to a track in question is heated by a laser beam, the generated heat is conducted to the track in question by heat transfer in the in-plane direction, with the result that the shape of the recording mark in that track is deteriorated under the thermal effect.
The cross-erase generated by the latter cause can be decreased by lowering the heat transfer in the in-plane direction. Therefore, a so-called “rapid cooling structure” is employed to decrease the cross-erase, in which structure a film having high thermal conductivity and/or high heat capacity is arranged in the vicinity of the recording film so as to promote the heat transfer in the perpendicular direction to the plane rather than that in the in-plane direction.
For example, in the conventional phase-change optical recording medium, a dielectric film (heat control film) having appropriate thermal conductivity is arranged between the recording film and a metal reflection film, and the dielectric film is formed relatively thin. The particular structure facilitates diffusion of the heat generated in the recording film to the reflection film so as to produce the effect of suppressing the heat transfer in the in-plane direction.
In the structure pointed out above, if the heat control film is made thinner, the heat transfer in the perpendicular direction to the plane can be promoted, which produces the effect of improving the cross-erase. However, if the heat control film is excessively thin, the heat transfer to the reflection film is caused simultaneously with heating the recording film by a laser beam in the recording process. As a result, temperature elevation of the recording film is rendered insufficient and, thus, a region that is heated to reach the melting point is made small so as to give rise to a problem that it is impossible to form a recording mark having a desired area. It should also be noted that, in the erasure process, the recording film is cooled simultaneously with the heating, with the result that the time period during which the recording film is retained within a temperature range in which the recording film can be crystallized is rendered insufficient. Thus, it is difficult to crystallize the recording mark so as to give rise to a problem that an erasure rate (erase rate or erasability) is markedly lowered.
To the contrary, where the heat control film is excessively thick, the problems can be eliminated in respect of the power margin of the laser beam in the recording process and the erasure rate. However, the in-plane heat transfer is promoted so as to cause the cross-erase problem easily. In addition, the cooling rate of the recording film is lowered, which causes the region that has been melted in the recording process to be recrystallized without being made amorphous, with the result that the mark formed is rendered excessively small.
Conventionally, a phase-change optical recording medium for suppressing the cross-erase is proposed (see Unexamined Japanese Patent Publication No. 2000-215516). The phase-change optical recording medium comprises a recording film, an upper protective film, an intermediate film, and a reflection film, which are formed in this order as viewed from the light incident side, in which characteristics of the material used for the intermediate film or the reflection film are defined to suppress the cross-erase. In this prior art, however, since the intermediate film uses a material having low thermal conductivity, it is difficult to rapidly cool the recording film, resulting in failure to produce a sufficient effect of suppressing the cross-erase.
As described above, it is known to the art to control the thickness and the thermal conductivity of the dielectric film arranged between the recording film and the metal reflection film. However, the conventional techniques were incapable of solving simultaneously all the problems including the sensitivity of the recording power, the cross-erase, the recrystallization and the erasure rate.
Also, a technique for lowering thermal damage to the recording film and the substrate is known, which uses SiC excellent in thermal conductivity for the second dielectric film in a phase-change optical recording medium comprising a first dielectric film, a recording film, a second dielectric film, and a reflection film, which are stacked in this order as viewed from the light incident side (see Unexamined Japanese Patent Publication No. 11-003538). However, the cross-erase is not studied in this prior art. It should be noted that it is impossible to achieve good recording characteristics and to suppress the cross-erase simultaneously by simply cooling the recording film.
Likewise, a technique for lowering thermal damage to the recording film is known, which uses SiC for at least a part of the first and second dielectric films in a phase-change optical recording medium comprising a first dielectric film, a recording film, a second dielectric film, and a reflection film, which are stacked in this order as viewed from the light incident side (see Unexamined Japanese Patent Publication No. 2002-269823). However, the cross-erase is not studied also in this prior art. Since the main purpose of this prior art is to improve overwrite characteristics by simply cooling the recording film and suppressing the thermal damage, it is impossible to suppress the cross-erase problem in this prior art.